The production of slabs, blooms and billets by the continuous casting process is well known. By this process such metal products are cast directly from molten metal instead of being produced by the previously employed multi-step process involving the casting of ingots, soaking and rolling the ingots into shapes.
In the practice of the continuous casting process, molten metal is continuously teemed into an open-ended mold from a superposed vessel, commonly called a tundish. Simultaneously therewith, the cast product is continuously withdrawn from the bottom of the mold in the form of a solid metal shell whose interior remains liquid until sufficiently cooled for complete solidification.
Ideal casting conditions require that molten metal be teemed to the caster mold at the same rate at which the solid metal shell is removed therefrom. Thus, a close control is normally maintained both on the rate of withdrawal of the cast shell from the mold and on the rate of supply of molten metal from the tundish to the mold to hold the level of molten metal in the mold substantially constant. Control of the molten metal supply to the mold in such installations is commonly effected by means of a sliding gate valve operably disposed at the pour opening of the tundish. These valves, which are available in several known forms, control flow to the mold by varying the size of the flow opening through the valve either by altering the positional alignment of an aperture in the movable gate plate of the valve with respect to an aperture in a stationary top plate or by changing gates containing apertures of varying sizes.
It is usually desirable in such applications to protect the poured metal stream against the possibility of atmospheric reoxidation and splashing as it flows from the tundish to the mold. This is particularly desirable in instances where aluminum-killed steel is poured wherein occlusions of aluminum oxide are formed by exposure of the metal stream to air and result in the rapid plugging of the metal pour passage. A well known practice for avoiding this problem involves enclosing the metal stream within an elongated pouring tube that extends from the discharge side of the gate valve into the interior of the caster mold. Such apparatus are disclosed in U.S. Pat. No. 3,459,346, granted Aug. 5, 1969 to B. Tinnes and U.S. Pat. No. 3,502,134, granted Mar. 24, 1970 to M. A. Orehoski.
Obviously, in order to prevent the ingress of air into the pouring tubes such as those described in these patents an effective air-tight seal must be provided between the interior and exterior of the tubes. When the pouring tube is properly sealed, initiation of teeming into the mold causes the molten metal level in the mold to rise and immerse the discharge end of the pouring tube whereupon any air present within the tube is rapidly exhausted therefrom by entrainment in the metal stream. As air is evacuated from the pouring tube, the metal level therein rises until the tube interior is ultimately filled.
Although the total evacuation of air from the interior of the pour tube as evidenced by the complete filling thereof with molten metal is desirable from the standpoint of the prevention of contact of the flowing metal with air, it has been discovered that, when the aspiration of air into the pouring tube is minimized by a substantially gas-tight seal about the juncture between the pouring tube and the gate valve, the rate at which metal flows through the tube into the mold significantly and unexpectedly increases thereby requiring the removal of the cast shell from the mold bottom to be accelerated in order to maintain the metal level in the mold constant. This phenomenon is referred to as "superspeed effect".
The occurrence of the "superspeed effect" is undesirable in the practical operation of a continuous caster because of the difficulties it presents in controlling the speed at which the cast shell is withdrawn from the mold or, alternatively, the deleterious effect created on product quality when shell withdrawl speed is not accurately controlled. These adverse results of "superspeed effect" stem from the fact that, in actual practice, there is frequent movement between the mating plates of the gate valve which continually changes the effectiveness of the seal between the gate valve and the pouring tube. As the effectiveness of the seal is reduced, there is a corresponding reduction in the rate at which metal flows into the mold and, conversely, when the seal becomes more effective the rate of metal flow increases. The overall result, therefore, is to impose a greater burden on the shell withdrawal control apparatus.
It is to the alleviation of the above-mentioned problem, therefore, that the present invention is directed.